Wireless telecommunications systems transmit signals to and from wireless terminals using radio frequency (RF) signals. A typical wireless system includes a plurality of base stations that are connected to the public switched telephone network (PSTN) via a mobile switching center (MSC). Each base station includes a number of radio transceivers that are typically associated with a transmission tower. Each base station is located so as to cover a geographic region known colloquially as a “cell.” Each base station communicates with wireless terminals, e.g. cellular telephones, pagers, and other wireless units, located in its geographic region or cell.
A wireless base station includes a number of modules that work together to process RF signals. These modules typically include, by way of example, mixers, amplifiers, filters, transmission lines, antennas and other appropriate circuits. One type of filter that finds increased use in wireless base stations is known as a microwave cavity filter.
Microwave cavity filters generally are formed from a machined, extruded, or cast body structure with enclosing walls to complete the filter cavity structure. The microwave signal generally enters the filter through an input connector and is coupled to an internal coupling pad or structure, and thus inserts it into the cavity structure of the filter. The filtered signal is extracted at the terminal end of the filter with another coupling pad or structure. Structures, such as resonators and/or tuning elements, that affect the electrical characteristics of the filter can be attached internally to the filter. Adjustment of the electrical characteristics of the filter can sometimes require the removal of a filter structural panel in order to gain access to the internally mounted elements.
Additional electronic circuitry, used with the filter is typically contained in one or more assemblies that are separate from the filter structure. This circuitry can include such things as inductors and capacitors to affect the electrical characteristics of the filter, low noise amplifiers (LNA's) and power amplifiers. These elements, being in separate assemblies from the filter body, require the use of connectors and cabling to couple them to the filter. The requirement of separate assemblies, connectors, and cabling can add expense, complexity, and source of undesired signal loss to the filter.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a filter with a simpler design and reduced number of components that provides lower costs, less complexity, more reliability, and easier tuning.